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Saturday, December 30, 2017

Electromagnetic waves of wavelength λ and frequency f travel at speed c in a vacuum. What describes the wavelength and speed of electromagnetic waves of frequency f / 2?



Question 3
Electromagnetic waves of wavelength λ and frequency f travel at speed c in a vacuum.

What describes the wavelength and speed of electromagnetic waves of frequency f / 2?


Wavelength                 Speed in a vacuum
A          λ / 2                                         c / 2
B          λ / 2                                         c
C         2λ                                            c
D         2λ                                            2c





Reference: Past Exam Paper – June 2013 Paper 13 Q24





Solution:
Answer: C.


The speed of EM waves in vacuum is c.
Speed c = f λ
Wavelength λ = c / f


The speed of all EM waves in vacuum is constant (= c).

If the frequency is now f/2

Speed in vacuum = c

New wavelength = c / (f/2) = 2c / f
 
Since λ = c / f,

New wavelength = 2 λ

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